1. Field of the Invention
The invention relates to the field of communication. More specifically, the invention relates to communication networks.
2. Background of the Invention
Optical networks were developed as a reliable means to carry voice traffic. Optical networks have evolved to also carry data traffic. The transmission standard for optical networks is synchronous optical network (SONET). SONET provides for automatic protection switching (APS) to protect traffic traveling over optical networks.
APS can be in the form of a ring switch (loopback), span switch, or path switch. SONET APS is typically done on a per SONET system basis. A ring switch occurs in a bi-directional line switched ring (BLSR). When a failure occurs in the BLSR, the affected traffic is ring switched to flow around the BLSR in a direction opposite of the original direction of the affected traffic around the failed span. A span switch can occur in an N-fiber BLSR (N being greater than 2) or on a linear connection, which is typically used to connect rings or in linear SONET systems. A span switch switches traffic from the failed fiber to another fiber that flows in the same direction. Span switching can be implemented as 1:1, N:M, or 1:N. Path switching can occur in a uni-directional path switched ring (UPSR). A path switch occurs when a receiving node detects a failure on the working path and switches to the protection path. The protection path typically flows in the opposite direction around the ring and is typically done on a unidirectional basis.
Despite the reliability of these methods of protection for optical networks, SONET APS is inflexible. The migration of optical networks from carrying voice to data has brought about a greater demand for higher transfer rates as well as a demand for diverse services. These services include multiple levels of quality of service (QoS). Unfortunately, multiple levels of QoS cannot be provided with SONET.